This invention is generally concerned with new developers, the use of such developers for causing the development of images in an electrophotographic system and more specifically, to the treatment of pigments used in toners primarily for providing a positive charge on such pigments.
The electrophotographic process is well known as illustrated in numerous prior art references including many patents. Generally, the process involves uniformly charging a photoconductive insulating surface which is placed on a conductive backing such as aluminum and subsequently exposing the photoconductive surface to a light image of the original to be reproduced. The photoconductive surface is prepared in such a manner so as to cause it to be conductive under the influence of the light image thus allowing the electrostatic charge form thereon to be selectively dissipated to thereby produce what is developed by means of a variety of pigmented resin materials specifically made for this purpose, such as toner. Such toner materials are electrostatically attracted to the latent image areas on the plate in proportion to the charge concentration contained thereon. Thus, for example, in areas of high charge of concentration, there is created areas of high toner density while in corresponding low charge images become proportionally less dense. The developed image can then be transferred to a final support material such as paper and fixed thereto for a permanent record or copy of the original.
Many processes are known for applying electroscopic particles or toner to the electrostatic latent image to be developed including, for example, cascade development described in U.S. Pat. No. 3,618,552, magnetic brush development described in U.S. Pat. Nos. 2,874,063, 3,251,706 and 3,357,402, powder cloud development described in U.S. Pat. No. 2,221,776 and touchdown development described in U.S. Pat. No. 3,166,432. In some instances, it may be desirable or necessary in the electrophotographic system to produce a reversed copy of the original. Thus, for example, it may be desired to produce a negative copy from a positive original or a positive copy from a negative original. Generally, this is referred to in the art as image reversal and in electrostatic printing, such image reversal can be effected by applying to the image a developer powder which is repelled by the charged areas of the image and adheres to the discharged areas. More specifically, toners containing positive charges are very useful and effective in electrophotographic reversal systems and in particular, in electrophotographic systems employing organic photoreceptors which in many instances, are initially charged negatively rather than positively, thus necessitating the need for a positively charged toner.
Reversal developers are described for example, in U.S. Pat. No. 2,986,521. Such developers comprise an electroscopic material coated with a finely divided colloidal silica. When this material is used in an electrostatic development system, the development of the electrostatic images on negatively charged surface is accomplished by applying the electroscopic material having a positive triboelectric relationship with respect to the colloidal silica. In U.S. Pat. No. 3,893,935 there is described use of certain quarternary ammonium salt as useful charge control agents for electrostatic toner compositions but not for pigments used in such compositions. According to the teachings of this patent, certain quaternary ammonium salts when incorporated into toner materials provide a particulate toner composition which exhibits relatively high uniform and stable net toner charge when mixed with a suitable carrier vehicle. This toner also exhibited a minimum amount of deleterious toner throw off. U.S. Pat. No. 4,079,014 contains a similar teaching with the exception that a different charge control agent is used namely a diazo type compound. In U.S. Pat. No. 3,749,670 electrostatic liquid developers containing carbon pigments treated with dyes having a positive charge is described.
Pigments such as carbon black, magnetite, color and other oxides, are used in a variety of products in order to impart color to these materials, and in particular, are used, for example, in toner materials for the purpose of providing color and controlling the charge on the toner by the charge contained on the pigment. Carbon black which is used as a pigment in electrophotographic toner has a rather substantial effect on the toner triboelectric properties and when used in electrophotographic ink, effects the direction and speed of pigment migration. When a carbon black material is used as a pigment, in general, in is negatively charged. Thus, when a positively charged toner employed in the development system, carbon black, which is a negative ingredient, adversely effects the developer triboelectric characteritics and the quality of the resulting image produced primarily because of the difficulty in attracting such a toner to the photoreceptor surface. Additives have been added to toners in order to cause them to charge positively, however, there is some disadvantages associated with these additives, such as additive migration, poor additive dispersion and poor toner particle-to-particle uniformity. This can cause wide distribution of electrical charge which in turn reduces the quality of the image developed and shortens the developer life.
Accordingly, there is a need for developer which contains a pigment such as carbon black which can be effectively used in a reverse system. More specifically, there is a need for a positively charged pigment which when used in a toner allows the production of high quality images over a long period of use.